EPOW - Ecology Picture of the Week

Each week a different image of our fascinating environment is featured, along with a brief explanation written by a professional ecologist.

15-21 August 2005

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The Boulder That Is A Tree

Rock Plant (Adenia spinosa), family Passifloraceae
southern Zimbabwe

Credit & Copyright: Dr. Bruce G. Marcot

Explanation:  Scrambling over large sedimentary boulders along the Limpopo River of Zimbabwe, southern Africa, I encountered an odd tangle of branches growing directly from this gnarled gray boulder.  Hard as a rock, this is no boulder.  It is a rock plant, Adenia spinosa.  

Growing best in acidic soils, rock plants are intolerant of frost but thrive in the heat of full sunlight.  The "rock" part is an above-ground tuber or caudex which can reach up to 1.8 meters (6 feet) in size.  The one photographed here is the largest I had ever seen.  

Rock plants produce thorns in the leaf axils, small cream-colored flowers in spring, and heart-shaped leaves.  They are native to the Northern Transvaal of South Africa and southern Zimbabwe, but have been introduced as exotic ornamentals elsewhere.  

Hiding behind this specimen in the photo is another tree of the arid woodlands of the region, Albizia or false-thorn (family Mimosaceae).


Next week's picture:  A "Not-So Virgin" Virgin Island

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